MOSCOW - Russians had thought such terrorist attacks were in the past.

But they are now digesting the official report that "terrorism" is the most likely explanation for an explosion on a popular intercity train that killed at least 26 people and injured more than 100 on Friday night.

"The main theory [being pursued by investigators] is that this explosion was caused by an explosive device planted by unidentified persons, or to put it plainly, a terrorist act," the head of the Russian Railways company, Vladimir Yakunin, told Russian state TV on Saturday night.

Russians have been here before. Ten years ago, as unrest in Russia's southern republic of Chechnya spiraled out of control, a series of still-unexplained apartment bombings killed almost 300 people in their sleep in Moscow and two other Russian cities.

Terrorists struck repeatedly in the next few years, killing hundreds in the siege of a Moscow theater, in an explosiion at a Moscow rock concert, in the downing two airliners in mid-flight, and five years ago, a horrific attack on a school in Beslan that ended with 330 people dead, most of them children.

"I just cannot bear the thought of those awful times coming back," says Irina Nurgayeva, a Moscow office worker whose sister was a hostage in the 2003 Nord Ost theater siege. "For years I was afraid every time I stepped out of my apartment. I was afraid at home too. Please, don't let it be terrorism again."

Almost 700 people were aboard the Nevsky Express, Russia's fastest train, which works the heavily-traveled Moscow to St. Petersburg route, when it was derailed just outside St. Petersburg by a bomb apparently planted beneath the tracks, according to police reports.

Three carriages, packed with people, were thrown off the tracks. Emergency workers were still sorting frantically through the wreckage late Saturday in fading hopes of finding alive some of the 18 people who remain listed as missing.

"Preliminary evidence suggests that it was an explosion of a homemade device, equivalent of seven kilograms of TNT," said Aleksander Bortnikov, head of the FSB internal security service.

A concerned-looking President Dmitry Medvedev addressed Russians on TV Saturday with a message of calm. "We need there to be no chaos, because the situation (in the country) is tense enough as it is," he said.

Though Moscow and other large Russian cities have not experienced major terrorist attacks for about five years, stability in the turbulent north Caucasus region has continued to unravel. Bombings, kidnappings, and political assassinations have grown increasingly common in Ingushetia and another republic, Dagestan, in recent months.

Many experts have been warning that it may be only a matter of time before extremist insurgents in the northern Caucasus return to attacking civilians in major Russian cities.

Andrei Soldatov, editor of the Internet journal Agentura.ru, which reports on security issues, says that the Nevsky Express was a logical target because the high-speed train was often used by Russian officials, several of whom do indeed appear to have been casualties of the bombing.

"It was not a simple attack, because a fast moving train is not an easy thing to destroy – there needed to be sharp timing – and explosives are much harder to come by these days," he says. "A few years ago one could obtain explosives readily, but nowadays it's a very difficult thing to do."

No one has yet claimed responsibility for Friday nights train bombing.

Though most eyes are turned to the seething north Caucasus as the source of the trouble, Mr. Soldatov and other experts warn that there is a much longer list of potential suspects.

"We know that Russian ultra-nationalists are growing in capabilities, and they have lately shown a readiness to move beyond racist attacks against persons of other nationalities to strike against the representatives of authority," he says. "But the best explosives experts are still to be found in the north Caucasus," he adds.